The present invention relates to an optical unit for dynamic wave forming with light modulator cells which are regularly arranged in cell arrays, said light modulator cells locally affecting partial light waves in a propagating light wave front. Each cell array is combined with a controller, which discretely controls the optical behaviour of the light modulator cells. This makes it possible to specifically adapt the form of the propagating light wave front locally so to shape a target form. In particular in high-resolution optical systems with large apertures, aberrations are disturbing and reduce the quality of the image presentation at the exit pupil of the system. With the aid of the optical unit, an adaptive optical system can for example be realised, in order to effectively compensate the aberrations in an optical imaging system with computer means.
Useful applications of the invention also include measuring and testing equipment in industrial production, for example if a process requires an adaptation of the focus to the form and condition of the material. Further, in optical networks with a flexible network structure, the invention can be used preferably to realise a variable assignment of connections to network hubs.
The invention can also be applied preferably in systems which holographically reconstruct objects, because holographic displays require display screens with apertures of a few decimeters in order to provide an adequately wide viewing angle for watching the reconstructions.
Further, both autostereoscopic and holographic systems often include means for finding eye positions and for directing and tracking light waves, which direct the modulated light for the autostereoscopic images or holographic reconstructions such that they exit the optical system at an oblique angle to the optical axis of the system, depending on the eye position.
Optical imaging systems which serve to present images or holographic reconstructions at high resolution and high luminous intensity often require several optical elements which have a wide aperture. This brings about locally varying aberrations in the beam path, where a combination of those known aberrations impairs the quality of the propagating light wave front at the point of exit of the system.
A ‘wave front’ is defined in the present document as an area of propagating light waves, on which during propagation through a propagation space all light points lie which have the same transit time from a transmitter, e.g. from a spatial light modulator SLM. A ‘cell array’ is defined in this document as the regular arrangement of modulator cells of a controllable spatial light modulator in a plane. It is not relevant for the subject matter of this invention whether the modulator cells are arranged in a matrix or in any other regular pattern, e.g. in a honeycomb pattern.
When large-area light waves propagate, there is the disadvantage that also ambient influences, such as fluctuations in temperature, humidity, composition and density of a medium in the propagation space, can dynamically change the portions of the above-mentioned aberrations in the wave front. If astronomical objects are observed with a telescope, for example, dynamic density fluctuations in the atmosphere can affect the optical condition of a received wave front. Those influencing factors change temporarily the refractive index and the aberrations of the optical system which modifies for example the image definition of an observed object dynamically. The blur is composed of the portion caused by the position displacement of the image in the viewing plane, and of a portion caused by the widening of imaged image points. The latter effect is described by a point spread function PSF, which defines the response of an optical system to a point light source or an object light point.
A similar problem is observed for example in a microscope. Also in this example it is desired to track the focus of the objective lens of the microscope quickly and/or to compensate local aberrations in the wave front.
In order to correct aberrations with computer means automatically and as promptly as possible, it is necessary to affect the form of the disturbed wave fronts adaptively with a minimum of mechanical movements involved. In the above-mentioned optical systems, the optical unit shall for example compensate the point spreading of as many as possible object light points and correct the extent of the object light points such that aberrations are eliminated as far as possible.
A similar problem is encountered in a system for the three-dimensional reconstruction of objects with a wave tracking for the propagating wave fronts. The wave tracking optically adapts the directions of propagation of the wave fronts to each current eye position of the corresponding observer eye, and tracks them if the eye position changes. In such a system, the light wave fronts propagate, depending on the observer positions, in propagation directions which are different from the optical axis of the system. This means that the directions of propagation run at different slanted angles to the optical axis and therefore cause aberrations with fluctuating portions.
The demands made on a holographic reconstruction as regards the capability of the wave fronts to generate interference additionally increase the necessity of reducing aberrations. To ensure that the modulated wave front reconstructs all light points of a three-dimensional scene as desired at the correct position, aberrations must be eliminated prior to the reconstruction. This requires for each possible eye position of an observer an aberration compensation which is adapted to that specific position. This problem can only be solved by a dynamically controllable wave front former.
Adaptive optical systems often take advantage of wave formers with a variably controllable, unstructured cell surface, i.e. a wave former with an optically continuous cell surface. For example, arrays with discretely moveable micro-elements are used whose inclination to the system axis can be individually adjusted electrically with the aid of motion means, and which are covered with a reflective membrane. Such a solution is disclosed for example in the publication WO 99/06856, titled “Microscope with adaptive optics system”, where a conventional microscope is provided with an adaptive optical system in the observation or illumination beam path. The adaptive optical system changes locally the phase and/or amplitude of the light of a spatial wave front in a defined manner on the one hand, and displaces the focus in the object space on the other.
The document discloses both transmission-type wave front formers based on LCD panels and reflection-type wave front formers with flexible membranes, which are moved electrostatically, piezo-electrically or with the help of bending elements.
Compared with a structured arrangement, an unstructured cell surface of a wave front former has the advantage that there is no risk of the occurrence of light diffraction at the cell surface, which would again impair the quality of the emitted wave front because of loss of light and/or interaction of various diffraction orders.
Major disadvantages of unstructured cell surfaces include a low control speed and the fact that only very large curvature radii can be achieved, which will hardly be narrower than R=1 m. This limits the dynamic range in many applications, so that unstructured wave front formers can only be used under certain conditions.
With the international patent application having the publication no. WO 2007/099456, titled “Wave front forming device”, the applicant has already sought protection for a two-dimensionally controllable, reflective wave front former which uses an array of discretely movable micro-mirror elements which have sizes of few micrometers. The wave front former has motion elements, which tilt or displace illuminated micro-mirror elements, or which perform a combination of those two movements, in order to set a structured reflection pattern mechanically. A hologram signal changes the reflection pattern by way of moving the micro-mirror elements at a fast pace and in synchronism of a video image frequency, thus generating a video sequence of hologram wave fronts in order to holographically reconstruct a moving scene in real-time. The reflection pattern reshapes the homogeneous wave form of incident light waves according to video hologram signals as a result of by locally different light reflections. The motion elements can move the micro-mirror elements by at least one full light wavelength.
The suggested device has the disadvantage that all movable mirror elements and a multitude of motion elements and the corresponding addressing means are all arranged together as an integrated circuit on a semiconductor substrate. This allows an effective former surface area of only few square centimeters, so that optical enlargement means are required for the wave fronts.
However, in contrast to an unstructured wave former, a wave former with a structured cell surface, in particular a spatial light modulator for phase modulation, is able to realise a high control speed and narrow curvature radii of few millimeters. As substantially smaller curvatures can thus be achieved, a structured wave former can realise a dynamic range which is ten times as large as that of an unstructured wave former, at a substantially higher spatial frequency.
Further, the international publication WO 2005/057250, titled “Variable focus system”, discloses a focus system with a multitude of moveable optical elements, where an electronic controller switches the focus positions of an optical system between different focus settings. The controller can either switch the focal length of the system between several image planes, or switch between different focus directions which differ from the optical axis. In a special embodiment, the focus system is used in conjunction with a video monitor which serves as a three-dimensional image projector in order to serially generate spatially staggered image planes, which image three-dimensional representations which are floating freely in space. The movable optical elements can be reflective elements, which can be tilted laterally in two dimensions, or refractive optical elements.
Further, with the non-published patent application DE 10 2007 005 8235, titled “Optical wave front correction for a holographic imaging system”, the applicant has already sought protection for a reconstruction system with a dynamic wave front former. The system images video holograms onto a display screen prior to reconstructing a scene. A position controller uses position information of current eye positions for controlling an optical wave tracking means such that the modulated light wave front appears at the desired eye position, irrespective of any changes in position. A controllable wave front former is disposed on the hologram side of the display screen. Based on position information that describes the current eye position, computer means adapt the optical behaviour of the wave front former such that the reconstructed scene exhibits the same geometry and optical quality, independent of the current position.
In contrast to the subject matter of the present application, the patent application DE 10 2007 005 8235 relates to using a unit for dynamic wave forming in a holographic imaging system—but not to its design features to suit manifold applications, for example for a direct-view display.
The international publication WO 2007/096687, titled “Diffraction gratings with tunable efficiency”, discloses a light modulator which takes advantage of a cell array with diffraction gratings in order to discretely locally modulate the luminous intensity and/or to deflect the light path. Each diffraction grating comprises controllable electrowetting cells, which are disposed in the light path of a backlight. This light modulator also operates in a transmission mode. The electrowetting cells have control electrodes, which affect the adhesion of capillaries with an electric field through the surface tension, and thus control the filling level or the surface shape of optical media and thus the optical transmittance. For this, the wetting cells form a capacitor where the space between the electrodes is filled partly with a hydrophobic liquid material, such as oil, and partly with water. At least one of the electrodes is coated with a hydrophobic material. Without an electric field applied, the hydrophobic liquid medium covers the electrode as a film, and with an electric field applied, the water displaces the hydrophobic medium, because the electric field eliminates the polarisation of the dipoles in the water surface.
The light modulator is for example intended for applications in graphical colour displays, which can be operated in a direct-view or imaging mode, with fast response times and short refresh rates. In addition, it is also applicable in an autostereoscopic display with sequential image presentation, for a controllable beam splitter and for deflecting light beams.
A disadvantage of structured wave formers, which is known as such, is that they do not only direct the light in one desired diffraction order. Additional, parasitic diffraction orders always occur in spatial periodicity. Depending on the optical system which uses the wave former, the parasitic diffraction orders exhibit various negative properties, such as loss in luminous intensity and/or interaction among the individual diffraction orders. This is why spatial filters, such as aperture masks or light traps for suppressing parasitic diffraction orders are often disposed in the optical path of optical systems with structured wave formers.
Wave formers with phase gratings also exhibit the above-mentioned drawback of parasitic diffraction orders.
The term ‘phase grating’ is used in this document for an optical diffraction grating where the control signals for the modulator cells locally affect the phase, but not the amplitude, of the passing light wave. As any other diffraction grating, a phase grating can be designed in the form of a transmission-type or reflection-type grating. Compared with a cell array with controllable amplitude gratings, a cell array with controllable phase gratings has the advantage that in the ideal case there is no loss in brightness compared with the incident wave front when controlling the modulator cells, which means that the luminous intensity does not decrease caused by the control process in an ideal controllable phase grating.
A diffraction grating splits incident light waves into emitted elementary waves, which interfere with each other and thus create a spatial frequency spectrum depending on the spacing of the grating. The higher the grating constant the sharper are global maxima imaged by the spatial frequency spectrum, and the more local maxima are generated at a lower intensity. This increases the resolution.
Now, the problem with wave front formers with a controllable phase grating is that the diffraction grating always deflects the coherent light emitted by the illumination means in multiple directions. If the illumination of the phase grating hits its surface at right angles, large portions of the incident light will be deflected by the grating into the diffraction order +1, and into the diffraction order −1, which is symmetrical to the first diffraction order. Non-diffracted light and other, parasitic diffraction orders can be suppressed with the help of a known structure of the diffraction grating, e.g. by way of destructive interference. In addition, if the illumination is incident at right angles, higher orders, such as the diffraction orders +2 and +3, can occur as well.
Because the luminous intensities of the diffraction orders +1 and −1 are usually identical, and because a portion of the light is also directed into the other diffraction orders as described, the usable portion of the light is substantially lower than 50% of the incident light with a phase grating so illuminated.
A similar situation can be observed if the phase grating is illuminated under a different angle of incidence. In that case, the majority of the incident light will be emitted by the grating in identical portions both with the diffracted light and with the first diffraction order. If the light wavelength λ is greater than the spacing of the grating, then no second order could exist, because the diffraction angle would be larger than 90°. In that configuration, disturbing diffraction orders can thus be eliminated without spatial filter means, and the available space behind the grating can be used for the propagation of the formed wave front. The grating period therein has about the magnitude of the wavelength.
However, controllable phase gratings with cell widths of a magnitude of the wavelength λ of visible light are not yet commercially available in the required large sizes.
Another problem of controllable phase gratings is caused by a discontinuous control within the spatial frequency. A wave front which is emitted by a controllable phase grating cannot be controlled continuously between two adjacent diffraction orders. If it is for example required that with a phase grating a modulated wave front is directed at an arbitrary position of an exit pupil in a space, then the wave front cannot reach positions which are located outside the spatial frequency spectrum of the grating.
The term ‘continuous’ is used in this document if the cell controller means are preferably digital computer means which control the controllable phase gratings with quantised phase values or which set them to quantised deflection angles by way of a known analogue-digital conversion. A ‘quasi continuity’ of a cell control is thus achieved with a precision of quantification levels, which depend on the resolution of the computer means.
Despite the above-described disadvantages of the structured wave formers, it is not possible to do without those structured wave formers, in particular phase modulators, for example of phase gratings, because of the drawbacks of unstructured wave formers discussed above.